# On the rank of incidence matrices in projective Hjelmslev spaces

Research paper by **Ivan Landjev, Peter Vandendriesche**

Indexed on: **30 Mar '14**Published on: **30 Mar '14**Published in: **Designs, Codes and Cryptography**

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#### Abstract

Let \(R\) be a finite chain ring with \(|R|=q^m\), \(R/{{\mathrm{Rad}}}R\cong \mathbb {F}_q\), and let \(\Omega ={{\mathrm{PHG}}}({}_RR^n)\). Let \(\tau =(\tau _1,\ldots ,\tau _n)\) be an integer sequence satisfying \(m=\tau _1\ge \tau _2\ge \cdots \ge \tau _n\ge 0\). We consider the incidence matrix of all shape \(\varvec{m}^s=(\underbrace{m,\ldots ,m}_s)\) versus all shape \(\tau \) subspaces of \(\Omega \) with \(\varvec{m}^s\preceq \tau \preceq \varvec{m}^{n-s}\). We prove that the rank of \(M_{\varvec{m}^s,\tau }(\Omega )\) over \(\mathbb {Q}\) is equal to the number of shape \(\varvec{m}^s\) subspaces. This is a partial analog of Kantor’s result about the rank of the incidence matrix of all \(s\) dimensional versus all \(t\) dimensional subspaces of \({{\mathrm{PG}}}(n,q)\). We construct an example for shapes \(\sigma \) and \(\tau \) for which the rank of \(M_{\sigma ,\tau }(\Omega )\) is not maximal.